A. Field of the Invention
The present invention relates to the field of lasers. More particularly, the field of the invention relates to lasers with mode locking.
B. Prior Art Background
Since the birth of laser more than three decades ago, the generation of short and ultrashort laser pulses has continued to be one of the most important and active areas of laser physics and engineering. Compact, efficient, reliable, ultrashort pulse lasers are very useful for many ultrafast photonic applications. Interest in solid state lasers lies in their long energy storage times and good spatial and spectral properties.
For example, neodymium (Nd)-doped lasers are simple solid-state lasers that can produce ultrashort pulses with high average powers (Repetition rate can trade with pulse energy). They can be pumped by compact semiconductor diode laser arrays with very high efficiency and have very good beam quality.
Conventional lasers typically contain a single laser medium in their resonators. The properties of the laser medium influences fundamentally the performance of the laser. Nd:YLF and Nd:YAG lasers are homogeneously broadened, with Nd.sup.3+ ions embedded in a crystalline background. The linewidths are .about.6 cm.sup.-1 and .about.12 cm.sup.-1, respectively. They have good phase coherence in the mode locking process, and easily produce coherent pulses especially by passive mode locking methods. However, their relatively narrow linewidths limit the pulse to widths of .about.2 ps.
Commercial lamp-pumped, actively mode-locked Nd:YLF and Nd:YAG lasers produce, respectively, .about.50 ps and .about.100 ps pulses with average powers of .about.20 watts. Somewhat shorter pulses have been produced from diode-pumped Nd:YAG and Nd:YLF lasers. Passive mode locking produces substantially shorter pulses. By additive-pulse mode locking, Kerr-lens mode locking, or using a semiconductor saturable absorber, Nd:YLF and Nd:YAG lasers can produce pulses .about.1.5-7 ps, which are limited basically by the linewidths of the laser media. In general, start-up of various passive mode locking processes is easy, when the intracavity laser power is above a certain level.
On the other hand, Nd:glass is inhomogeneously broadened, with different Nd.sup.3+ ions experiencing different local fields in an amorphous structure, has a wider linewidth, .about.200-300 cm.sup.-1, and is capable of generating and amplifying less than 100 fs pulses. But, very often, good phase coherence among the lasing axial modes is difficult to attain. This results in partially coherent pulses, difficult start-up in the passive mode locking process, and the gain bandwidth of Nd:glass cannot be fully utilized.
Although a Nd:glass laser was first mode-locked in 1966, continuous-wave (CW) mode-locked Nd:glass laser had not been realized until 1984. 7 ps pulses have been obtained from a CW, actively mode-locked Nd:phosphate glass laser pumped by an argon laser. Later, laser-diode-pumped, actively mode-locked Nd:glass lasers have been demonstrated, and 5-20 ps pulses have been obtained. Note that in the CW free-running state, the lasing bandwidth is about 7-15 cm.sup.-1 ; when fully mode-locked, this bandwidth may allow generation of 2 ps pulses. It was observed, however, that often the available bandwidths can not be fully mode-locked to produce coherent ultrashort pulses and a Fabry-Perot etalon was needed to limit the bandwidth. Passive mode locking has been employed to produce much shorter pulses. By additive-pulse mode locking, 300-500 fs pulses have been generated. Using a semiconductor saturable absorber, 130 fs pulses were generated. In Nd-doped fiber lasers, owing to the very strong integrated nonlinearity in fiber, pulses as short as 70 fs have been generated. In general, however, passive mode locking of a Nd:glass laser is much more difficult than the Nd:YLF and Nd:YAG lasers, using similar mode locking mechanisms. Additive-pulse mode locking of Nd:glass is less stable and Kerr-lens mode locking of Nd:glass has not been demonstrated yet. This has been attributed to the difficulty of start-up of mode locking in Nd:glass lasers because of the presence of a large number of lasing axial modes and consequently a shorter coherence time of the initial spiking pulse. Experiments showed that the less inhomogeneously broadened the laser is, the easier the start-up and stable operation of the passive mode locking process.
Some lasers contain more than one laser media, but they lase independently. Some other laser systems consist of an oscillator and amplifiers. One example is using Nd:YLF or Nd:YAG in a mode-locked oscillator and Nd:glass in amplifiers. But the mode-locking performance of the oscillator does not improve.